Communication systems in general, and paging systems in particular, have attained widespread use. In such paging systems, transmitted call signals are used to call selected receivers for the purpose of transmitting information from a base station to the selected receivers. Modern paging receivers have achieved multifunction capabilities through the use of microprocessors which allow the receivers to respond to information containing various combinations of tone, tone and voice, or data messages in a variety of modes. This information may be transmitted using several paging coding schemes and message formats.
Some considerations governing the successful operation of a paging receiver relate to the portability of the receiver, battery saving, available memory, radio spectrum availability, and fast response time. Equally important, however, is reliability, one aspect of which is the device's ability to continue to function properly after sudden mechanical impacts and shocks (e.g., dropping the unit onto a hard surface).
Modern selective call receiver units generally include relatively thin printed circuit boards, housings which are typically made of a plastic type material, and fragile electronic components. The plastic housing's front and back planes typically have a low frequency response to sudden impact resulting in relatively large deflections which will result in secondary impacts with the internal printed circuit board components. Certain ones of these components are fragile in nature; i.e. some components are constructed of quartz, ceramic, and silicon. Each of these components themselves have a natural frequency response to impact that amplifies the incoming shock.
The relatively thin printed circuit boards themselves have a low frequency response to shock resulting in large deflections. They also have a variable frequency of vibration response across the printed circuit board due to the variation in components' weight across the board. Furthermore, vibrations of the printed circuit board are undamped. The natural vibration frequency response of the housing may be approximately 390 Hz resulting in a deflection of 0.07 inches. The frequency of vibration of the printed circuit board will range from 200-300 Hz resulting in a deflection of 0.14 inches. Modern low volumetric selective call receivers do not permit tolerances of a significant amplitude to accommodate such deflections. As a result, sudden mechanical shocks result in unit failures. Large impacts, whether primary or secondary, create detached or broken solder joints in integrated circuits, ceramic filters, and other components. The excessive printed circuit board deflections overstress and fatigue solder joints resulting in failure.
The current method of providing shock isolation within a selective call receiver is to place one or more pieces of shock isolating material in selected areas. Unfortunately, this approach has provided a limited amount of shock isolation in a single direction only and does not solve all of the problems described above.
Other sources of device failure stem from the fact that the interior of modern selective call receiver housings contains a large volume of air. This may result in the formation of condensation which can adversely affect the electrical operation of the device. Furthermore, it is possible for contaminants such as water to enter the housing and occupy these regions likewise causing device failure.
Thus, what is required is an apparatus for shock isolating the selective call receiver and its constituent parts by occupying the normally void internal portions of the receiver housing and altering the frequency response to mechanical shock of the receiver unit and its constituent parts so as to minimize their deflection.